On the photodissociation dynamics and spectroscopy of atmospheric molecules

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Copyright: Lee, Kin Long Kelvin
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Abstract
In this thesis, we detail an investigation into the photodissociation dynamics and spectroscopy of three atmospherically relevant chemical species: acetone, ethane (C2H6) and nitrous oxide (N2O). A systematic study into the photodissociation mechanisms of acetone has been carried out across its first electronically excited state, covering a wavelength range of 230 - 312nm. The photodissociation of acetone comprises primary and secondary dissociation mechanisms, involving one and two C−C bond ruptures. At wavelengths longer than 306nm, we have discovered a novel dissociation pathway, involving a barrierless dissociation to CH3CO + CH3, reproduced by a statistical prior model. We estimate this mechanism will account for 83% of CH3 formed by absorption of actinic radiation. The remaining 17% is accounted by dissociation in the T1 state, which dominates at wavelengths shorter than 306nm. This reaction channel was readily modelled by a statistically adiabatic-impulsive (SAI) model. At wavelengths shorter than 263 nm, secondary dissociation of acetone can occur. The CH3CO fragment from the primary step undergoes dissociation, yielding CH3 + CO. We refute the roaming mechanism believed to occur in this wavelength region, and the only two pathways observed are the primary and secondary dissociation reactions. High level electronic structure calculations have been undertaken to determine the ionisation energies and dynamics of ethane. Using the HEAT345(Q) procedure, we determine adiabatic ionisation energies for the 2Eg and 2A1g states to be 11.52 and 11.56 eV respectively, with an estimated uncertainty of 0.025 eV. The photoelectron spectrum of C2H6 has also be simulated using a quadratic model vibronic coupling Hamiltonian, comprising the two lowest-lying electronic states of C2H+6 . The linear coupling model reproduces the spectral envelope, and a quadratic model is required for quantitative agreement with the reference spectrum. The O 1D photofragment yield from the photodissociation of N2O, N2O−N2O and N2O−O2 have been measured over the wavelength range 206 - 216 nm. we observe no difference in the UV absorption properties between these three species, contrary to theoretical simulations. A new photodissociation channel has also been discovered in the N2O−O2 complex, producing O 3P fragments. A spin-flip mechanism is proposed to account for this reaction.
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Author(s)
Lee, Kin Long Kelvin
Supervisor(s)
Kable, Scott
Jordan, Meredith
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Publication Year
2017
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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